SYNTHESIS AND CHARACTERIZATION OF HYDROTRIS(PYRAZOLYL)BORATE DIHYDROGEN HYDRIDE COMPLEXES OF RHODIUM AND IRIDIUM/

Protonation of TpM(PR3)H-2 (M = Rh, Ir) complexes with HBF4 . Et2O or [H(Et2O)(2)][B(Ar)(4)] (Ar = 3,5-(CF3)(2)C6H3) affords cationic comple xes which exhibit a single hydride resonance at all accessible tempera tures in the H-1 NMR spectrum. Formulation as fluxional dihydrogen/hyd ride complexes is indicated by short T-l(min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H-H bond leng ths of 0.88-1.11 Angstrom in the iridium complexes and 0.73-0.92 Angst rom in the rhodium complexes depending on the relative rate of the dih ydrogen relational motion. In the case of the iridium complexes, parti al substitution of the hydride positions with deuterium or tritium res ults in large temperature-dependent isotope shifts and resolvable J(H- D) or J(H-T) coupling constants. Analysis of the chemical shift and co upling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rathe r than the dihydrogen site. This analysis also provides the limiting c hemical shifts of the dihydrogen and hydride ligands as well as the (1 )J(H-D) coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.